Staircase electrode-wall configuration for MHD generators

Abstract

A staircase electrode-wall configuration for a MHD generator which uses fd dynamic means to protect the electrodes from substantial damage due to arcing and chemical attack. The electrode walls of a MHD generator duct have periodic conductor/insulator elements where channel divergence or convergence is accomplished in a stepwise fashion. The electrode is situated within the insulator of each element to be backward-facing so as not to be directly exposed to the hot gas flow. The turbulence which exists at the backward-facing electrode moves the arc spot about the surface to prevent severe local damage. Chemical attack is inhibited by a laminated electrode surface of a low corrosive material and the turbulence which aids in cooling the electrode face. Additional cooling may be obtained by a purge jet at the electrode face.

What is claimed is: 
     
       1. An electrode configuration for an MHD generator in which a hot plasma flows along a duct having opposing electrode walls, said hot plasma being subjected to a strong magnetic field, comprising a plurality of periodic electrode elements forming said electrode walls, each of said electrode elements being in the form of a backward-facing step having an insulator portion and an electrode portion, said electrode portion being situated so as not to be directly exposed to said hot plasma and so as to have the electric field generated coact with said magnetic field to essentially balance the convection force at the exposed surface of said electrode portion, and each of successive ones of said electrode elements along the axis of said hot plasma flow being offset to form a steplike divergence for said duct. 
     
     
       2. An electrode configuration as recited in claim 1 wherein each of said electrode elements comprise: (a) an insulator portion having a slope portion and a backward-facing step portion; and   (b) an electrode portion being situated in said backward-facing step portion so as not to be directly exposed to said hot plasma.   
     
     
       3. An electrode configuration as recited in claim 2 wherein said electrode portion comprises: (a) a highly electrically conductive portion having a cooling duct passing therethrough; and   (b) a laminate of a high strength non-corrosive material to form the face of said electrode, the thickness of said laminate being such as to not significantly impede the flow of current between said opposing electrode walls.   
     
     
       4. An electrode configuration as recited in claim 3 further comprising means for introducing a purge jet of inert gas at said electrode face without disturbing the recirculation area formed by said hot plasma flow at said backward-facing step. 
     
     
       5. In an MHD generator of the type in which a hot plasma flows along a duct having opposing electrode walls through a strong magnetic field, an improved electrode arrangement for forming said walls, which comprises: (a) a plurality of periodic electrode elements with each electrode element being in the form of a backward-facing step and having an insulator portion and a conductor portion;   (b) said conductor portion being embedded in said insulator portion so that only a single surface of said conductor portion is uninsulated and uncovered, said uncovered surface being on the downstream end of said element, facing downstream, and oriented normal to the longitudinal axis of said duct;   (c) said insulator portion having an upward sloping surface extending from the upstream end of said element to a surface parallel to the longitudinal axis of the duct, said parallel surface extending to the downstream end of said electrode element;   (d) said electrode elements being arranged to provide a step-depression from the backward-facing surface of the upstream electrode element to the upward sloping surface of the downstream electrode element; and   (e) said arrangement thereby providing that said conductor portion is not directly exposed to said hot gas flow and also providing a turbulent region of three dimensional flow and a recirculation region two-dimensional flow formed by said hot gas flow downstream of the step-depression.   
     
     
       6. The electrode arrangement as recited in claim 5 wherein successive electrode elements are offset to form steplike changes in the opposing electrode walls. 
     
     
       7. The electrode arrangement as recited in claim 5 wherein successive electrode elements are offset to form a steplike divergence in said opposing electrode walls. 
     
     
       8. An electrode arrangement as recited in claim 5 wherein said conductor portion includes: (a) a cooling duct passing therethrough; and   (b) a laminate of a high strength non-corrosive material to form the uncovered face of said conductor portion, the thickness of said laminate being such as to not significantly impede the flow of current between said opposing electrode walls.   
     
     
       9. An electrode arrangement as recited in claims 5 or 7 further including means for introducing a purge jet of inert gas at said conductor surface without disturbing the recirculation region formed downstream of the step-depression.